This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. Unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Colloidal semiconductor nanoparticles present very interesting optical and electronic properties due to the ease with which the emission wavelength and the electronic states can be controlled. They can further be assembled by a simple drying process into randomly close packed and even highly ordered solids. Because they are synthesized as colloids, they are covered by surfactants that provide a steric repulsion against aggregation. This organic layer has the additional benefit of electronically passivating the dangling orbitals of the metal cations, therefore removing carrier traps and helping radiative recombination of excitons. However, the organic layer also poses a significant barrier to electron transfer between particles. Typically, organic saturated hydrocarbons presents a negative electron affinity, and thus, would lead to typical energy barriers of the order 5 eV for typical semiconductors. In practice, as determined by tunneling experiments on alkane monolayers, the average barrier height is expected to be significantly lower, of the order 2 eV reduced in part by image potentials. Nevertheless, this is a high barrier in the context of traditional semiconductor heterostructures where barriers of 0.5 eV are the norm. Consequently, initial experiments investigating transport with colloidal quantum dot arrays primarily demonstrated their insulating properties.
At present, conductivity in quantum dot arrays has been achieved by a liquid medium procedure that involves the extraction of the original ligands in the films and their replacement by shorter passivating ligands. For CdSe nanocrystals, of interest for visible light emission, the highest mobility reported by this approach is about 10−2 cm2/V/s. For a smaller band gap material, PbSe, of more interest as an infrared emitter, mobilities of 10−2 cm2/V/s up to 1 cm2/v/s have been reported. This corresponds to hopping times between 1 nanosecond (ns) and 10 picoseconds (ps) between nanocrystals. This characteristic can also can be expressed in terms of a coupling energy of order 0.01 to 1 meV. This is still small compared to Coulomb repulsive energy or polydispersivity (100 meV). As a result, the organic-inorganic composite nanomaterials made by this approach are intrinsically Mott-insulators.
It is a fundamental challenge to achieve “metallic” behavior in such close-packed arrays. It would also open the door to applications requiring high mobility and high currents, such as electrically driven lasers or photovoltaic devices. Applications of the materials to both cases can be easily motivated. Indeed, the quantum dot lasers based on heteroepitaxy provide insufficient dot density to significantly improve the performance over the existing quantum well laser. With close-packed colloidal dots, dot density is easily increased by 100-fold. The bigger challenge is to draw a high and ambipolar current in the colloidal dot systems. With photovoltaics, such high currents are not required, but it is clear that higher mobilities will be in general very helpful. To overcome the difficulties described hereinbefore due to the organics, a barrier material that is all inorganic has been attempted in the prior art, but has been rather unsuccessful using liquid solution methods. A challenge with liquid solution is to avoid self-nucleation in the liquid phase, as well as pore-clogging in the nanoparticle arrays and to have enough diffusion of the bulky solvated molecules into the array.